The present invention relates generally to a new type of film forming material having unique photochemical properties. Non-scattering, optically clear films formed from the new materials can be easily prepared. They allow light-induced generation of optical anisotropy (photo-induced dichroism and birefringence) therein and of topological surface structures, e.g. such as surface relief gratings (SRG). The material comprises a complex prepared from at least two components: 1) an anionic or cationic polyelectrolyte and 2) an oppositely charged cationic or anionic photosensitive low molecular weight compound or a second, oppositely charged polyelectrolyte carrying photosensitive side chains, the low molecular weight compound and the photosensitive side chains of the second polyelectrolyte, respectively, having the ability to undergo E/Z isomerization or to participate in a photo-cycloaddition or in a photo-rearrangement reaction or another reaction capable of generating optical anisotropy in the material upon irradiation. The homogeneous material based on this complex readily forms films, preferably on solid substrates or between two such substrates from water/alcoholic or organic solvents.
It is known that amorphous and liquid crystalline polymers containing azobenzene or other photoactive moieties such as stilbenes, cinnamates, coumarins in side chains or main chains can be used for the induction of anisotropy by photoorientation (K. Ichimura, Chem. Rev. 2000, 100, 1847; A. Natansohn et al., Chem. Rev. 2002, 102, 4139; V. Shibaev et al., Prog. Polym. Sci. 28 (2003) 729-836; X. Jiang, et al., WO 98/36298). Azobenzene derivatives are also known for their ability to form SRG when being exposed to gradient light field (A. Natansohn et al., supra)
Different types of azobenzene containing materials were used for optical anisotropy and/or SRG generation. In one approach (“guest-host” systems), this was attained by mixing of photochromic azobenzene derivatives, e.g. 4-[4-N-n-hexyl-N-methylamino-phenylazo]-benzoic acid or modified Direct Red 1 azodye with readily available polymer PMMA as a matrix (J. Si et al., APPL. PHYS. LETT. 80, 2000, 359; C. Fiorini et al., Synthetic Metals 115 (2000), 121-125). However, the effects to be observed are rather weak, due to low dye loading caused by dye-polymer segregation. Relatively high loading of the photochromic material in the polymer matrix could be observed with specially synthesized dyes, which allow avoiding a dye-polymer segregation (C. Fiorini et al., see above). But in such systems the photo-induced dichroism was not stable, and the SRG formation was not effective (up to 50 nm deep). Relatively stable birefringence has been induced only when commercially available Direct Red 1 was introduced into very high-Tg poly(ether ketone). It is unknown whether SRGs can be generated in the latter system. Such materials were used for the recording of orientational holograms.
Better results have been obtained by chemically binding azodye compounds to a polymeric material. The material is characterized by covalent bonds between the photoactive units and the polymeric backbone. In addition to the fact that the results observed are much better than in the “guest-host” approach, such polymer materials normally have good film forming properties. However, environmentally non-friendly organic solvents have to be used. Often the solubility of the polymers is a problem which is hardly to overcome. Special synthesis is required to manufacture such functional polymers from commercially available chemicals, and consequently, they are expensive. Moreover, the purification of the polymers is a difficult problem as well.
Moreover, a specially synthesized monomeric azobenzene derivative has been found which is able to form glassy films (V. Chigrinov et al., 1106•SID 02 DIGEST; V. A. Konovalov, et al., EURODISPLAY 2002, 529; W. C. Yip et al., Displays, 22, 2001, 27). In films of these low molecular weight glass forming compounds optical anisotropy was induced by irradiation with linearly polarised light. Usually monomeric dye compounds have poor film forming properties and tend to crystallize.
A layer-by-layer (LBL) dipping procedure has been employed to obtain films for photo-induced orientation and SRG formation (see e.g. A. M.-K. Park et al, Langmuir 2002, 18, 4532; Ziegler et al., Colloids and Surfaces, A 198-200 (2002), 777-784; V. Zucolotto et al., Polymer 44 (2003), 6129-6133). In such systems, readily available polyelectrolytes and low molecular weight azodyes possessing at least two ionic groups, azobenzene containing bolaamphiphiles, ionenes or polyelectrolytes covalently substituted with azobenzene moieties are used. In the typical procedure, a substrate is alternately immersed for about 10-20 min in an aqueous solution of a cationic polyelectrolyte, such as poly-DADMAC, and an anionic azobenzene containing compound, respectively. Each immersion results in the formation of a monolayer on the substrate surface with typical thickness of about 1 nm. Numerous repetition of this procedure results in a multilayer film. About 150 layers are required to obtain a reasonable thickness of the resulting layer. Films up to 700 layers can be produced. SRGs with an amplitude of up to 120-140 nm can be generated, wherein a photoinduced orientation of the azobenzene moieties can be observed. The procedure is tedious and time consuming. Moreover, rather thick films are necessary for the inscription of deep SRG, and such films are difficult to obtain.
Another approach using H-bonds between the polymeric backbone and the photochromic compounds has been employed (E. B. Barmatov et al., Polymer Science, Ser. A, Vol 43 (3), 2001, 285). In this way, films with the ability for photoorientation were obtained.
In these concepts, the components are bound to each other by Coulomb attraction or H-bonds. Similar attraction is possible between oppositely charged ionic moieties in solution. The interaction of polyelectrolytes with dyes in dilute solutions has been studied (W. Dawydoff et al., Acta Polym. 1991, 42, 592). Recently, complexes of polyelectrolytes with another, oppositely charged polyelectrolyte containing a diazosulfonato moiety in the side chain were fabricated as a solid material (A. F. Thunemann et al., Macromolecules 1999, 32, 7414; 2000, 33, 5665). The molecular photochemistry and light-induced subsequent physical processes of these materials such as photoorientation and photo-induced diffusion, were not investigated.
In summary, a multiplicity of chemical systems making use of the photochemical properties of photochromic azobenzene dyes has been developed during the past few years. Such compositions may form films, which allow introduction of optical anisotropy and/or the generation of surface relief structures therein. However, despite the intense search for effective and readily available compositions, they are all connected with certain disadvantages as outlined above.